Rechargeable Li-ion cells are key components of the portable, entertainment, computing and telecommunication equipment required by today’s information-rich, mobile life. Polymer electrolytes formed through dissolving salts into polar and high-molecular-weight polymers are the subject of intensive research because of their potential for use as the solid polymer electrolyte (SPE) in rechargeable lithium batteries.1-6 Both the salt and the neat polymer are solids and the complex-forming reaction can be expressed as follows:7
n m
n MX RY
RY
mMX +(− −) →( ) •(− −) (1-4)
where (–RY–) corresponds to the repeat unit of polymer. The kinetics of eq (1-4) are unfavorable, even when the complex is stable. The most commonly used method to dissolve or suspend both the MX salt and the host polymer is dissolving them in a common solvent and then removing the solvent, producing the solvent-free polymer electrolyte.8 Obviously, the dissolving reaction will be thermodynamically favorable (negativeΔG°) only if the Gibbs energy of dissolving the salt into polymer is large enough to overcome the lattice energy of the salt, thus three parameters are important for the controlling these salt/neutral molecule interactions: (a) electron pair donicity, (b) acceptor number, and (c) an entropy term. The electro pair donicity is corresponded to the ability of the solvent donating electrons to solvate the cation as Lewis acid, thus the incorporated polymer should exhibit donor site such as oxygen, sulfur, or nitrogen either in the backbone or in the side chain. The acceptor number represents the possibility for anion (base) solvation. For instance, PEO, a polyether, exhibits strong donor and its donicity is close to 20, additionally, ethers are very poor acceptors because they lack hydrogen bonding for anion salvation.9 Thus, PEO can effectively solvate cation possessing bulky delocalized counter anions such as I-, ClO4-, BF4-or CF3SO3-which require little or no salvation.
The use of these electrolytes for high-energy-density batteries and other solid state
electrochemical devices spurred considerable interest in the ion-transport properties of these materials.10-15 Besides the advantage of flexibility, polyelectrolyte can also be cast into thin films and since thin films while minimizing the resistance of the electrolyte also reduces the volume and the weight, use of polymer electrolytes can increase the energy stored per unit weight and volume. As shown in Figure 1-2, the polyelectrolyte serves as a medium to transport the ions in the cell. In addition, a separator isolating the anode from the cathode electronically can be ceramic or polymeric separator as using liquid electrolytes. Both functions, ion conduction and separation, can be realized in a single thin membrane when polymer electrolytes are used.
The goal for the SPE research is the development of highly ionic conductive (ca. 10–4 S/cm), dimensionally stable, and flexible SPE materials under ambient condition. Since Wright reported that poly(ethylene oxide) (PEO) can be the candidate for use in the SPEs at 1975, it has became one of the most studied materials16 and polymer electrolytes were proposed for batteries in 1978 because they exhibited the advantages of solid state electrochemistry with the ease of processing inherent to plastic materials.17 Since that time, the number of contributions to the field of SPEs related to PEO has grown enormously, these PEO based polyelectyrolytes were prepared through numerous physical and chemical procedures including non-covalent blending,10-15 covalent copolymerization,18-21 and grafting22 for studying their interaction mechanism and for improving their flexibility and chemical and physical properties without detrimentally affecting the ionic conductivity. The ionic conductivity σ can be roughly expressed by the following equation:
∑
and the ion mobility, respectively, indicating that the fraction of “free” ions is an important parameter, a high degree of dissociation of the salt in the polymer is needed for obtaininghighly ionic conductive polyelectrolyte. High Li+transference number is also needed, i.e., a high ratio of the charge transfer which has been an important subject of research in recent years.23-24 The molecular dynamic simulation shown in Figure 1-3 indicated that the Li+ions are complexed to PEO through approximately five ether oxygens of the PEO chain, and thus mobility of the cations is decreased considerably,25 implying that the mobility of the Li+cation is related to the motion of the complexed PEO chain. In summary, the polymer required for the SPE application should possess that (1) high concentration of polar (basic) groups and (2) low cohesive energy and high flexibility to solvate the salt effectively, resulting high ionic conductivity.26-30
References
1. Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nature 1998, 394, 456.
2. Gray, F. M. Polymer Electrolytes (RSC Materials Monographs, The Royal Society of Chemistry, Cambridge, 1997).
3. Scrosati, B. (ed.) Applications of Electroactive Polymers (Chapman & Hall, London, 1993).
4. Bruce, P. G. (ed.) Solid State Electrochemistry (Cambridge University Press, Cambridge, 1995).
5. Fenton, D. E.; Parker, J. M.; Wright, P. V. Polymer 1973, 14, 589.
6. Ratner, M. A. in Polymer Electrolytes Reviews－1 (eds MacCallum, J. R.; Vincent, C. A.) 173–236 (Elsevier Applied Science, London, New York, 1987).
7. Ratner, M. A.; Shriver, D. F. Chem. Rev. 1988, 88, 109.
8. LaNest, J. F.; Cheradame, H.; Dalard, F.; Deroo, D. J. Appl. Electrochem. 1986, 16, 75.
9. Gutmann, V. The Donor Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978.
10. Druger, S. D.; Ratner, M. A.; Nitzan, A. Solid State Ionics 1983, 9/10, 1115.
11. Armand, M. B.; Chabango, J. M.; Duclot, M. J. in Fast Ion Transport in Solids (eds Vashishta, P., Mundy, J. N.; Shenoy, G. K.) 131–136 (North-Holland, Amsterdam, 1979).
12. Chiu, C. Y.; Yen, Y. J.; Kuo, S. W.; Chen, H. W.; Chang, F. C. Polymer 2007, 48, 1329.
13. Gorecki, W. Solid State Ionics 1998, 28, 1018.
14. Chiu, C. Y.; Hsu, W. H.; Yen, Y. J.; Kuo, S. W.; Chang, F. C. Macromolecules 2005, 38, 6640.
15. Chiu, C. Y., Chen, H. W., Kuo, S. W. & Chang, F. C. Macromolecules 2004, 37, 8424.
16. Wright, P. V. Br. Polym. J. 1975, 7, 319.
17. Armand, M.; Duclot, M. French Patent 1978, 7832976.
18. Cho, B. K.; Jain, A.; Gruner, S. M.; Wiesner, U. Science 2004, 305, 1598.
19. Epps, T. H.; Bailey, T. S.; Waletzko, R.; Bates, F. S. Macromolecules 2003, 36, 2873.
20. Singh, M.; Odusanya, O.; Wilmes, G. M.; Eitouni, H. B.; Gomez, E. D.; Patel, A. J.; Chen, V. L.; Park, M. J.; Fragouli, P.; Iatrou, H.; Hadjichristidis, N.; Cookson, D.; Balsara, N.
P. Macromolecules 2007, 40, 4578.
21. Zhang, H.; Kulkarni, S.; Wunder, S. L. J. Phys. Chem. B 2007, 111, 3583.
22. Gavelin, P.; Jannasch, P.; Wessle´n, B. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 2223.
23. Armand, M. Solid State Ionics 1994, 69, 309.
24. Bruce, P. G.; Vincent, C. A. Faraday Discuss. Chem. Soc. 1989, 88, 43.
25. Müller-Plathe, F.; Van Gunsteren, W. F. J. Chem. Phys. 1995, 103, 4745.
26. Vincent, C. A.; MacCallum, J. R. In Polymer Electrolyte Reviews; Mac Callum, J. R., Vincent, C. A., Eds., Elsevier: London, 1987.
27. Armand, M. B.; Chabagno, J. M.; Duclot, M. J. In Fast Ion Transport in Solids; Duclot, M.
J., Vashishta, P., Mundy, J. N., Shenoy, G. K., Eds.; North-Holland:
New York, 1979.
28. Shriver, D. F.; Papke, B. L.; Ratner, M. A.; Dupon, R.; Wong, T.; Brodwin, M. Solid State Ionics 1981, 5, 83.
29. Paper, B. L.; Ratner, M. A.; Shrever, D. F. J. Phys. Chem. Solids 1981, 42, 493.
30. Paper, B. L.; Ratner, M. A.; Shrever, D. F. J. Electrochem. Soc. 1982, 129, 1694.
Figure 1-2. Schematic illustration of a lithium rocking chair battery with graphite and spinel as intercalation electrodes and its electrode reactions.
Figure 1-3. Schematic of the segmental motion assisted diffusion of Li+ in the PEO matrix.
The circles represent the ether oxygen atoms of PEO.
1.4. Introduction to Proton Exchange Membrane (PEM) Applied in Direct Methanol